1. Field of The Invention
This invention relates to an improved high voltage generator for providing energy to a computed tomography (CT) scanning x-ray tube. More particularly, the apparatus includes a high voltage stabilization circuit for reducing fluctuations in x-ray energy production.
2. Prior Art
In order to generate x-radiation for CT scanning, an x-ray tube including an anode and cathode electrode is positioned in close proximity to a patient and x-radiation from that tube is transmitted through the patient to an array of x-ray intensity detectors. The intensity data is used in a reconstruction algorithm to provide a density mapping of a patient cross-sectional area. To generate the x-rays, electrons from the cathode are accelerated to the anode due to the force exerted on them by an electric field between the two electrodes. This field is proportional to an electric potential difference applied between those electrodes.
Fluctuations in the voltage separation between cathode and anode result in fluctuations of the electric field and therefore fluctuations in the speed with which the electrons strike the anode material. As the electrons strike the anode, their kinetic energy is transferred to x-radiation energy and heat energy which is dissipated by the anode. Variations in electron kinetic energy result in variations of x-ray energy emitted by the x-ray tube. It is apparent, therefore, that variations or fluctuations in the electrical potential difference between the cathode and the anode result in fluctuations or variations in the energy of x-radiation emitted by the x-ray tube.
The reconstruction of a cross-sectional density mapping by a CT scanner requires accurate intensity information regarding the x-radiation which was passed through the patient. To reconstruct these images it is advantageous to have non-varying accelerating potentials across the x-ray tube to achieve stable x-ray generation and therefore accurate reconstruction images. To generate x-rays the accelerating potential is provided by a high voltage generator which must provide stable voltages of the order of one hundred thousand volts.
One prior art constant potential generation technique is disclosed in the U.S. Pat. No. 3,325,645 entitled "X-Ray Tube System With Voltage and Current Control Means" which has been assigned to the assignee of the present invention. The apparatus embodied in that prior art disclosure includes an alternating high voltage source whose output is, first coarsely adjusted, rectified and then smoothed or filtered with a capacitor. This filtered signal is then transmitted to an x-ray tube cathode and provides the accelerating potential for electrons in the x-ray tube.
Fine adjustment in the accelerating potential is achieved by a feedback control circuit which modulates the amplitude of the signal reaching the filtering capacitor. By modifying the unfiltered high tension voltage in response to the voltage appearing at the x-ray tube the accelerating potential is stabilized. An increase in the filtered signal results in a change in the impedance of the feedback control circuit which tends to reduce the unfiltered signal. Conversely a decrease in the filtered signal results in a change in the impedance of the feedback circuit which tends to add or increase that unfiltered signal. This negative feedback control reduced variations in accelerating potential by reducing the ripple in the filtered high voltage signal.
While achieving substantial commercial success especially in the application for which it was designed, x-ray diffraction, the prior art apparatus disclosed in U.S. Pat. No. 3,325,645 contains some disadvantageous features when applied to CT. One such feature is the utilization of the filtering capacitor to smooth a pulsating signal. Such a capacitor charges to a voltage of many thousands of volts and can retain a substantial amount of stored energy. In CT applications this stored energy can present a safety hazard to both service personnel and to the x-ray tube.
The prior art voltage generator has a limited range of feedback control. If the unfiltered voltage from a transformer secondary in the prior art circuit deviated from a certain range the feedback control circuit could no longer stabilize the accelerating potential. For stabilization beyond this limited effective range a primary transformer control circuit is required to return to the proper range the output from the secondary transformer to the filter capacitor. This limitation added complexity to the stabilization circuit. Finally, the commercial embodiment of the '645 patent operated at too low an accelerating potential and current range for optimum CT use.